U.S.-backed Syrian forces entered a Syrian town near the Turkish border that has been held by Islamic State (IS) militants for more than two years, after a three-week-long battle with the terror group around the town.

Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab forces, advanced towards Manbij on Thursday with aerial support from the U.S.-led coalition, the Syrian Observatory for Human Rights reported.

“Clashes are taking place on the southern side of the city,” Rami Abdulrahman, head of the Syrian Observatory told VOA via phone from Britain.

“Two things are preventing the SDF to take full control of the city — civilians who are being used by Daesh (IS) and booby traps that the group has planted all over the city,” he said.

IS fighters suspected inside city

Abulrahman, whose group has reporters on the ground, told VOA that at least 1,000 IS fighters are still inside the city, fighting Kurdish-led forces with light and mid-range weapons.

The U.S.-led coalition expected IS militants to fight back in Manbij until the last minute, given its strategic importance to them.

FILE - Men, who the Democratic Forces of Syria fighters claimed were Islamic State fighters, walk as they are taken prisoners after SDF advanced in the southern rural area of Manbij, in Aleppo Governorate, Syria, May 31, 2016.

Manbij is a key waypoint on an IS supply line between the Turkish border and the extremist group's self-proclaimed capital, Raqqa, which is also under intense pressure from a Syrian government offensive. If Manbij is captured, it will be the biggest strategic defeat for IS in Syria since July 2015, when it lost the border town of Tal Abyad.

“We’re around the city and we’re coming in, and we’ve cut the lines of communication into it and the noose is getting tighter around the city,” U.S. Army Colonel Chris Garver told reporters Wednesday from Baghdad.

Earlier this week the SDF opened a corridor and some residents were able to escape the town toward safer areas.

“Daesh was trying hard to keep us in Manbij,” one fleeing resident told VOA.

“We had been waiting for the [SDF] forces for two months to come to our aid,” he said.

Liberating Manbij

Manbij, Syria

At least 20,000 people are still held by IS militants inside the town, local sources said.

The campaign to liberate Manbij began three weeks ago and at least 520 IS fighters were killed, according to Kurdish military officials.

Manbij had been one of the major supply lines for IS militants in areas they control in northern Syria. Local activists say pushing back IS from the town would be a significant setback for the terror group in Syria.